The present invention generally relates to techniques for processing information. More particularly, the invention provides a method and apparatus for converting CELP frames from one CELP based standard to another CELP based standard, and/or within a single standard but a different mode. Further details of the present invention are provided throughout the present specification and more particularly below.
Coding is the process of converting a raw signal (voice, image, video, etc) into a format amenable for transmission or storage. The coding usually results in a large amount of compression, but generally involves significant signal processing to achieve. The outcome of the coding is a bitstream (sequence of frames) of encoded parameters according to a given compression format. The compression is achieved by removing statistically and perceptually redundant information using various techniques for modeling the signal. Hence the encoded format is referred to as a “compression format” or “parameter space”. The decoder takes the compressed bitstream and regenerates the original signal. In the case of speech coding, compression typically leads to information loss.
The process of converting between different compression formats and/or reducing the bit rate of a previously encoded signal is known as transcoding. This may be done to conserve bandwidth, or connect incompatible clients and/or server devices. Transcoding differs from the direct compression process in that a transcoder only has access to the compressed signal and does not have access to the original signal.
Transcoding can be done using brute force techniques such as “tandem” which has a decompression process followed by a re-compression process. Since large amount of processing is often required and delays may be incurred to decompress and then re-compress a signal, one can consider transcoding in the compression space or parameter space. Such transcoding aims at mapping between compression formats while remaining in the parameter space wherever possible. This is where the sophisticated algorithms of “smart” transcoding come into play. Although there has been advances in transcoding, it is desirable to further improve transcoding techniques. Further details of limitations of conventional techniques will be described more fully throughout the present specification and more particularly below.